This invention relates to a method of sea bottom prospecting through the generation of submarine acoustic pulses having time characteristic and spectrum adapted to a high-resolution prospecting, and a tuned array of paraboloidal electroacoustic transducers to carry out such method.
The described apparatus can be advantageously used for the prospecting of the sea bottom structure and the subsurface of the sea bottom both in civil engineering and archaeological research and in geophysical applications as well. Furthermore, it can be used for the prospecting of lakes or in any natural body of water. Generally the apparatus of this invention can be advantageously used in all of applications of submarine acoustics in which a high acoustic power with high resolution is needed in the low and medium frequency range (0.1 to 15 kHz) not covered by the traditional acoustic devices like SONAR.
Most of the traditional sources for the sea prospecting have only a few of the necessary requirements to generate high-resolution prospecting pulses because the frequency band thereof is rather limited and the high frequency harmonic content is poor. Moreover the cavitation pulse associated to the primary pulse further reduces the feasibility of a good performance of the sources. The criteria for judging of the quality of a seismic sea-source are essentially based upon the duration of the acoustic pulse, the frequency content and the repeatability thereof, and the primary/cavitation pulse ratio (see G. Parkes and L. Hatton, The Marime Seismic Source, D. Reidel edition 1986, Dordrecht, Holland).
As for the first two characteristics, i.e. duration of the acoustic pulse and frequency content, the ideal signal would be a socalled "spike" or pulsed signal which can be represented by a Dirac delta function, i.e. a very short pulse in which all frequencies are contained (see M. B. Dobrin, Introduction to Geophysical Prospecting, Mc Graw-Hill, 1960, New York). The cavitation pulse (see A. Prosperetti, Physics of Acoustic Cavitation, Rendiconti Societa Italiana Fisica, 1984, Varenna), is a drawback always present in seismic underwater prospecting which, however, can be eliminated in a digital signal processing, for example, by deconvolution techniques (see E. A. Robinson and S. Treitel, Geopnysical Signal Analysis, Prentice-Hall, Inc., 1980, Englewood Cliffs). Nevertheless a prospecting pulse is desired which is similar as far as possible to the ideal signal before any digital processing of the seismic signal. This is the necessary condition to obtain high-resolution "acoustic images", thus minimizing cost and waste of time during the signal processing.